Power conversion systems are used in supplying power to a grid, in driving electric motors, and in other applications in which electrical power must be converted from one form to another. Typically, the power converter is constructed using electrical switches actuated in a controlled fashion to selectively convert input power to output power of a desired form such as single or multi-phase AC of a controlled amplitude, frequency and phase to drive an AC motor according to a desired speed and/or torque profile, often in the presence of varying load conditions. Many motor drives include an initial AC to DC power conversion stage that receives multiphase AC voltage from a power source and employs selectively actuated electrical switching devices (e.g., IGBTs, etc.) to perform a controlled rectification to provide DC power on a bus. In many applications, the DC bus is then inverted by another set of controlled switches to provide an output AC (single or multiphase) to drive an AC load, such as an electric motor or a power grid. The initial AC to DC conversion stage may include inductors in the input phase lines to allow generation of DC voltages higher than the peak AC input voltages, thereby facilitating provision of output/load power at any suitable levels. Often, however, the phase voltages from the AC power supply are unbalanced. Even small source voltage imbalance in an AC power converter can result in large discrepancies between phase currents, depending on the filter impedances at the input to the converter. Large differences in the converter phase currents, in turn, can result in stresses to switching components (e.g., IGBTs) of corresponding phase branches in the AC to DC conversion. Accordingly, designers of power conversion systems must either oversize the switches to accommodate source imbalance situations or operate the converter below rated output to prevent switching component damage or degradation. Moreover, voltage imbalance situations may exacerbate the generation of undesired harmonic content by the converter, particularly second harmonics that exceed tolerable levels. U.S. Pat. No. 7,355,865 to Royak et al., assigned to Rockwell Automation Technologies, Inc., provides for control of second order harmonics in voltage imbalance situations by using current regulation in the stationary reference frame, the entirety of which patent is hereby incorporated by reference herein. However, this approach may be limited to certain AC line applications to prevent instability. Accordingly, there remains a need for improved apparatus and techniques for power conversion system phase current balancing for source voltage imbalance situations.